1. Field of the Invention
The present invention generally relates to integrated circuit technology, and more particularly, to an integrated inductor structure.
2. Description of the Prior Art
Inductors are broadly applied in electrical circuits, such as transformers, power converters, radio frequency (RF) circuits, and microwave circuits. A major trend of circuit design is to incorporate as many circuit components into integrated circuit form as possible, whereby fabricating costs of the entire circuitry can be minimized. In general, so-called integrated inductors may be fabricated by CMOS standard process, for example logic baseline process, or other special processes, such as RF baseline process. Each process has its own advantages and disadvantages.
Because integrated inductors fabricated by logical baseline process use the same process as other portions of the circuit, and the cost of logic baseline process is relatively low, the total circuit fabrication costs can be lowered. However, metal layers used for realizing inductor coil patterns have limited thickness. (Take 0.13 μm CMOS logic baseline process as an example, wherein the top metal layer has a thickness of approximately 8-10 k Å.) Such a limited thickness may result in excessive resistance. Moreover, the parasitic capacitance is difficult to reduce, which leads to inductors having a low Q factor that affects the efficiency of the inductor, especially in high-speed RF applications. On the other hand, integrated inductors fabricated by a special process such as RF baseline process, may substantially decrease resistance due to much thicker metal layer that can be used for fabricating inductor coil patterns. (Take 0.13 μm RF baseline process as an example, wherein the top metal layer has a thickness of more than 20 k Å, and can even be as high as 30 k Å.), which in turn may result in inductors having a better Q factor and a larger inductance. However, special processes are more complicated and often require more photoresist layers. For these reasons, the entire circuit may have a higher fabrication cost.